No, it does not get a boost if it's holding a mega stone.
>Knock Off's power has been increased from 20 to 65, and its PP increased from 20 to 25. In addition, it will deal 50% more damage if used on a Pokémon holding an item other than a Mega Stone or a Giratina holding a Griseous Orb. Sticky Hold does not prevent this bonus.
Hope I helped.